Pulp treating process



May 14, 1935 J. D. RUE 2,001,268

PULP TREATING PROCESS Filed Oct. 18, 1933 Patented May 14, 1935 UNITED STATES PULP TREATING PROCESS John D. Rue, Niagara Falls, N. Y.,

Hooker Electrochemical Company,

assignor to New Yo k,

N. Y., a corporation of New York Application October 1a, 1933, Serial No. 694,171 In Canada September 21, 1932 10 Claims.

My invention has for its object to produce a bleached pulp having, in a very large degree, the desirable strength characteristic of the original brown stock. It is well known that the brown color of wood pulp is due mostly to the lignin and other intercellular materials and thatthe strength is mostly due to the cellulose, which is naturally white (e. g. cotton fiber). The bleaching of pulp therefore involves the problem of removing the lignin without injuring the cellulose. The preliminary digestion processes have for their object to separate the fibers by removing the bulk of the lignin. These processes, in general, involve cooking with steam and strong alkalies or compounds of sulphur. This is rough treatment for cellulose and generally results in a considerable destruction of cellulose and impairment of its strength. The digestion process cannot be carried too far, therefore, without serious loss. This step leaves the pulp brown in color and with a considerable proportion of lignin still remaining. Subsequent bleaching operations have for their object to remove more of the lignin and oxidize the residues to colorless compounds. The processes hitherto used for this purpose generally have been such as to further injure the cellulose. Where maximum strength is desired, regardless of color, the sulphate'process is used and the stock is intentionally under-cooked, producing what is known as kraft pulp. This, of course, is dark brown in color. For this reason, only brown kraft pulp has heretofore been available. By my process, however, it is possible to produce bleached pulp from any stock, including kraft, with none of the desirable characteristics of the original stock seriously impaired and with some of them actually improved.

Heretofore it has been the practice in'the chlorination of pulp to start with insufficient chlorine for complete chlorination of the lignin and allow it to exhaust itself. In contra-distinction to this practice, my process consists in starting with a very substantial excess of chlorine and stopping the reaction very definitely and positively before complete exhaustion. The reasons for this procedure, objectssought and advantages gained will be understood from the following:

It is known that absorption of chlorine in a pulp suspension is very rapid for the first few minutes, then slows down very sharply and thereafter is almost imperceptible. I have discovered that the period of rapid absorption corresponds to chlorination of lignin, and that if the reaction be stopped after three to five minutes, substantially all the llgnin has been selectively chlorinated with no appreciable attack upon the cellulose. It will now be understood that when chlorine is allowed to react to 10 exhaustion a long period must elapse during the greater part of which little is happening except injury to the cellulose. In practice this amounts to from one to two hours. This prolonged chlorination has heretofore been thought necessary in order to avoid waste of chlorine whenthe chlorination products are subsequently washed out. I have discovered that it is this prolonged contact with chlorine that injures the cellulose.-

In my process, therefore, I stop the reaction after three to five minutes, or in less than onetenth of the customary time of reaction, thus protecting the cellulose from injury. In order to secure adequate chlorination in this exceedingly brief time, I start with a great excess of chlorine, usually 25 to 100 per cent, over that required for chlorination of the 'chlorinatable constituents, such as the lignin content. This excess, aided by vigorous agitation, ensures the desired reaction within the stated time. Instead of washing, which would be comparatively slow and would waste the excess chlorine, I stop the chlorination by the addition to the mixture of an alkali, preferably calcium hydroxide. This neutralizes the acid already formed, dissolves the chlorinated lignins and combines with the excess chlorine to form calcium hypochlorite. This hypochlorite then oxidizes the coloring matter in known manner, which, of course, effects a partial bleaching. There therefore result the following advantages:

(a) A great gain in time.

(b) A simplification, reduction in size and cheapening of the reaction apparatus.

(c) A' marked improvement in quality of the chlorinated pulp.

(d) A partial bleaching in the chlorination step which greatly facilitates the subsequent hypochlorite bleaching.

For purifying pulp by removal therefrom of such impurities as and lignin residues and by transformation of colored components into colorless products, thus leaving the pulp white, prior investigators have established that purification may be eifected to advantage by a combination of a treatment with chlorine with a treatment of hypochlorite. Whenchlorine is brought into contact with water it may react to effect either chlorination or oxidation, depending upon the acidity or alkalinity conditions present: If the solution be acid (as obtains when chlorine is added to water alone) the tendency is toward chlorination, while the tendency is toward oxidation if the chlorine is in solution wholly in the form of a hypochlorite in neutral or alkaline solution.

While the process of the present invention is susceptible to various applications in the art by combination thereof with other operations, it consists essentially in the following steps in the following sequence: introducing chlorine, in the form of gaseous or liquid chlorine or of an aqueous solution thereof, preferably, continuously and at a uniform rate, into a stream of a low density pulp suspension flowing at a uniform rate, quickly and thoroughly mixing the chlorine and the pulp, passing the thoroughly mixed mass without material agitation through a space calculated to allow the elapse of a period of time for the desired reaction between chlorine and pulp but not for the complete exhaustion of the chlorine, adding, preferably continuously, to the stream of mixture an alkali in quantitysumcient to combine with the chlorine as such and with the acids contained in the mixture and to render the pulp suspension neutral or alkaline, thoroughly mixing the alkali with the pulp whereby to insure the aforesaid neutralization of acids and the combination of the alkali with the unconsumed chlorine as hypochlorite, reacting the alkali and the pulp suspension until the hypochlorite content is substantially exhausted (a noticeable bleaching of the pulp usually being effected in this step) washing soluble by-products from the resulting chlorinated and partially oxidized pulp suspension and, with or without a thickening of the latter to any desired consistency, treating the pulp with a hypochlorite bleaching agent by any of the known methods, thereafter washing soluble by-products from the resulting bleached pulp. In the carrying out of this process the chlorine is introduced into the stream of low density pulp suspension, and is caused to act thereon, while the latter is confined within a closed space in such manner that escape of chlorine gas from the pulp suspension is avoided; this confinement of the pulp suspensionchlorine mixture continuing until the aforesaid alkali addition and neutralization shall have been effected. This necessarily involves maintaining the pulp suspension-chlorine mixture under superatmospheric pressure.

As stated above it has been found that the above-described method involving application of chlorine as a step in the bleaching, or delignification and bleaching, process has certain advantages over known bleaching methods, in that thereby there is obtained a high degree of whiteness in the pulp while economlzing in chlorine consumption; it is possible, also, to produce a bleached pulp of improved strength in that it is possible to cook to higher yields (that is, undercook the raw cellulosic fibrous material) and still have a pulp that can be delignified satisfactorily and bleached to a high degree of whiteness without substantial sacrifice in strength. The equipment necessary for effecting the above-described chlorination stage is simple, anda minimum of labor and of supervision are required for the continuous first stage bleaching. Thereby there are attained most of the advantages of a three-stage bleaching at very little if any additional consumption of bleach and with the elimination of one of the three washings which a three-stage bleaching requires, thus eliminating expensive washing equipment and loss of fiber incident to a stage of washing. It is an advantage in economy, also, to be able to effect chlorination wholly at low consistency of the pulp and without the necessity for costly concentration equipment. A result of major importance resides in the fact that it is thereby possible to carry out the chlorination step, down to and including the termination thereof by neutralization with alkali, in apparatus devoid of moving parts.

As stated above, it has been found, as e. g. in the case of the chemical wood pulps, that the absorption of chlorine takes places very rapidly especially during the first one to three minutes of treatment of the pulp, and that thereafter the absorption proceeds at a decreasing rate of speed. The quantity actually absorbed within a given time depends upon several factors as e. g. the character of the pulp, the initial concentration of the chlorine, the temperature. etc. The following table will illustrate the absorption of chlorine by three different types of pulps:

Table I Bulphite Sulphate Kraft Chloyrin? adiied o pu p 3. 67 3.96 9.

Chloi ine absorbed in l minuto- 00 of pulp 2. 33 3. 39 7. 49

of Cl; added 64.0 85. 6 83. 2 Chlorine absorbed in 3 minutesof pulp 2. 79 3. 49 7. 69

% of Oh added 76. 0 88. 2 86. 4

It has been stated above that, according to present practice, when direct chlorination is used and the chlorine allowed to become exhausted, as much as one or two hours may be required for the process, in which case equipment must be provided, in commercial operations, to contain the corrosive pulp mass for that period of time. Such An important essential to the process is the continuous additions of the chlorine to the pulp, each in uniform and suitably proportioned flow, followed by prompt and thorough mixing. Failure to secure rapid distribution'of the chlorine throughout the pulp mass results in areas of high chlorine concentration and other areas of low chlorine concentration. Under such conditions it is obvious from such data as those given in Table I that a portion of the pulp would be overchlorinated and another portion would be under-chlorinated and that a lack of desirable uniformity of quality of the finished bleached product would result. To insure rapid mixing the pulp should be of low consistency, i. e., not to exceed 6%, prelerably around 3% or 4%.

As has been set out hereinbefore, the chlorination treatment is terminated not by exhaustion of the chlorine but rather by addition to the chlorine-pulp suspension of an alkali which may be a caustic alkali such as caustic soda or an alkaline earthmetal compound of alkaline reaction such as lime. This alkali may be added in any convenient form, for instance as a solid or a suspension in liquid, or a solution, it being essential only that the agent be added to the chlorine-pulp mixture before the chlorine has been exhausted, and that thereafter a period of time be provided for reaction between the pulp and hypochlorite thus produced. This step is accomplished, in accordance with the invention, by introducing the alkali into the flowing stream of pulp continuously and at a regulated rate and in an amount sumcient to efiect the transformation aforesaid.

The ensuing period of intimate contact between the alkaline agent and the pulp sufilces to dissolve solubilized chlorinated products and as well to effect a noticeable bleaching of the pulp, whereby the succeeding washing operation readily removes the impurities leaving the pulp in a desirable state for the application of a hypochlorite bleach. When it is desired to develop in the finished product qualities of softness and absorbency it is preferable to use a dilute solution of caustic alkali as the neutralizing agent, and to carry out the; longed contact between alkali and pulp at substantially boiling temperature. Otherwise, the neutralization may be efiected by means of milk of lime or an alkaline sulphite or bisulphit'e, preferably at an elevated temperature.

The invention will be more particularly described with reference to the accompanying drawmg, according to which a a The single figure is a side elevational view, partly in cross-section, of an embodiment of the apparatus according to the present invention.

In the drawing: l is the chlorination tube, 2 is the fluid injector, and 3 is a porous plug closing the mouth of the injector 2. Injector 2 is admittedinto the tube I through the removable insert block 4, which latter fills the space of the T opening 5 in tube I. 6 is a cover plate for T opening 5. 1 represents the neutralization tube, provided with an injector tube 8 through the opening 9. I0, III are helical worms fixed within tubes I and I, respectively, by end spiders II, II. The said worms are provided with a plurality of projecting pins l2, I2, projecting from the worm shafts in staggered relation. I3 is the reaction chamber, being a cylindrical vessel of cross-sectional dimension relatively large with respect to that of tubes I and I and reduced in. dimension by frusto-conical ends to correspond to the tubes I and 1. Chamber I3 is connected with tube I by the L connection It, and to the tube I by the conduit l5 constituting therewith 'a gas-tight chamber. Parts may, as indicated by admixture of the chlorine with the pulp, the pins l2 adding to the turbulence of the flow worm by repeatedly breaking the stream. The resulting mixture then passes as a continuous stream into and through the reaction chamber l3 and thence through the neutralizing tube I where a stream of milk of lime or other alkaline solution is introduced into the stream of mixture at regulated rate and in the desired amount. which, in any event, is suflicient to neutralize acids present and to combine with the chlorine present as such, by means of the injector tube 8 and is brought into thorough admixture with the alkali by passage about the worm in. Tubes l and I andchamber l3 with their connections I4 and I 5 constituting a closed space which in operation is completely filled with pulp suspension, and the chlorine being introduced into the stream of pulp suspension in the manner described, the pressure is substantially maintained throughout the passage through tube I and chamber I3 and into tube I, thereby obviating any loss of gaseous chlorine from the suspension by passing ofi. Under the specific conditions hereinafter described it has been observed that a pressure of about20 to 30 pounds per square inch normally develops. This pressure on the pulp suspension is advantageous for the chlorination process in 'view of the fact that thereby the chlorine dissolves in the watermore rapidly and Period in Folding enball mill Mullen durance Tm Min. Pta/IbJream Double folds Gram Z) 1. 61 264 50. 8 40 l. 92 696 51. 4 60 I 2 10 1175 38. 4 2. 08 1455 39. 8 2. 00 1342 37. 2

was suspended in water to form a 3% pulp suspension. This latter was sent through the chlorination tube at the rate of one and a half tons per hour. Chlorine gas was added in the proportion of 1.5 pounds per 100 pounds of pulp (dry weight), and the chlorine and pulp were passed through the reaction chamber designed to cause chlorine and pulp to remain in the reaction relation for 3 minutes, whereupon there was added to the stream milk of lime in the proportion of pounds of hydrated lime per 100 pounds of pulp (dry weight), and thoroughly admixed therewith. Thereafter the mixture was delivered into and through a conventional reaction chamber (not shown) of such capacity as to allow for a period of 10 minutes for the complete use and exhaustion of the calcium hypochlorite so formed, the stream of mixture thence being passed to a conventional washer where water-soluble products were removed. The pulp was then suspended in water to form a 5% pulp suspension, was treated with an alkali hypochlorite in the proportion of 1.8 pounds per 100 pounds of pulp (dry weight),

in the known manner, to exhaustion of the agent,

past the Beat Folding en- DQ110 35 Mame durance Tm Min. Pic. lit/ream Double folds gram 20 1. 63 X 45. 8 40 1. s4 7 me 41. o l, 89 1444 38. 0 2. '04 1621 34. 0 100 2. 13 3156 32. 3

bleached pulp made from identical unbleached kraft pulp, the one by my process and the other by the ordinary commercial process.

a a. g a s gs s 2 t! li s 8 5 3 '8 3 :3 ,q 9 k ,8 GE 0 E 3 ea 2' P" O a a see ta s to ,D m R 2400 1. 74 2. l2 1. 48W 2. 55 2. 85 2. 26 7200 3. 14 3. (B 2. 17 9600 a.1a 3.00 2 l2 1M0 3. 22 3. 08 2. 04 24(1) 101 93 59 4300 103 83 47 7200 97 74 36 9600 8) 65 35 12000 78 61 30 Double folds 2400 200 200 100 48% 1M I 7200 1850 16m 400 9600 2200 2230 400 12000 3400 2000 425 It will be seen that in every case my process gives results of marked superiority, notwithstanding the fact that the pulp bleached by y process was noticeably whiter than the other.

It will be obvious that since my process is applicable to any pulp 'at any stage, it is capable of superseding, if desired, the later stages of the digestion step.

This application is a continuation-in-part application of my application Serial No. 547,755, filed June 29th, 1931.

V I claim:

1. Process which comprises introducing into a water suspension of pulp chlorine in substantial excess of the amount required for chlorination of the non-cellulosic constituents of said pulp, promptly and thoroughly mixing the chlorine with the pulp, reacting the chlorine with the pulp to a point at which 20 to 50 per cent of the chlorine remains unreacted and thereupon introducing an alkali into the reaction mixture in amount sufiicient to neutralize the mixture and combine with the unreacted chlorine, thoroughly mixing the alkali with the pulp, reacting the resuiting hypochlorite with the pulp and washing.

2. Process which comprises adding continuous- 1y to a water suspension of pulp of low consistency chlorine in substantial excess of the amount required for chlorination of the lignin "in said pulp. each in uniform and suitably proportioned continuous flow, promptly and thoroughly mixing the combined chlorine and pulp streams, causing themixedmasstoreacttoapointat which 20 to 50% of the chlorine remains unreacted, thereupon adding to the pulp an alkali in amount suiiicient to neutralize acids present proportioned continuous flow, mixing the combined alkali and pulp streams and causing the residual available chlorine in the form of hypochlorite substantially to become exhausted.

3. Process as defined in claim 1 characterized in that the washed pulp is subjected to a second stage of bleaching in which the agent is a chlorlte'.

4. Process as defined in claim 2 characterized in that the pulp is washed and subjected to a sec- 0nd stage of bleaching in which the agent is a hypochlorite.

, 5. The process as defined in claim 1 characterized in that the chlorine is introduced into the water suspension of pulp and mixed and reacted therewith while the latter is confined in a closed space substantially completely filled with the same.

6. Process which comprises introducing into a water suspension of pulp chlorine to an amount 25 to 100 per cent in excess of that required for chlorination of the lignin in said pulp, promptly and thoroughly mixing the chlorine with said pulp, reacting the chlorine with the pulp to a point at which 20 to 50 per cent respectively of the chlorine remains unconsumed and thereupon introducing an alkali into the reaction mixture in amount sufiicient to neutralize the mixture and combine with the unconsumed chlorine, thoroughly mixing the alkali with the pulp, reacting the resulting hypochlorite with the pulp and washing the pulp.

7. Process which comprises introducing into a water suspension of pulp chlorineto an amount 25 to 100 per cent in excess of that required for chlorination of the lignin in said pulp, promptly andthoroughly mixing the chlorine with the pulp, reacting the chlorine with the pulp approximately three to five minutes, whereby the lignin is substantially all chlorinated without appreciable effect upon the cellulose, and thereupon introducing an alkali into the reaction mixture in amount sufficient to neutralize the mixture and combine with the unconsumed chlorine, thoroughly mixing the alkali with the pulp, reacting the resulting hypochlorite with the pulp and washing the pulp.

8. Process which comprises introducing into a water suspension of pulp chlorine in such amount that, after promptly and thoroughly mixing the chlorine with the pulp and reacting for approximately three to five minutes 20 to 50 per cent of the chlorine remains unconsumed, whereby the lignin is substantially all chlorinated without appreciable eiTect upon the cellulose, and thereupon introducing an alkali into the reaction mixture in amount sufiicient to neutralize the mixture and combine with the unconsumed chlorine, thoroughly mixing the alkali with the pulp, reacting the resultant hypochlorite with the pulp and washing the pulp.

9. Process which comprises introducing into a water suspension of pulp chlorine to an amount 25 to 100 per cent in excess of that required for chlorination of the non-cellulosic constituents of said pulp, promptly and thoroughly mixing the chlorine with the pulp, reacting the chlorine with therein and to combine with unconsumed chlorine the alkali and pulp, each in uniform and suitably the pulp to a point substantially short of complete exhaustion of the chlorine as such and thereupon introducing an alkali into the reaction mixture in amount suflicient to neutralize the mixture and combine with the unconsumed chlorine, thoroughly mixing the alkali with the pulp, reacting the resulting hypochlorite with the pulp and washing the pulp.

10. Process which comprises introducing into a water suspension of pulp chlorine to an amount 25 to 100 per cent in excess of that required for chlorination of the non-cellulosic constituents of said pulp, promptly and thoroughly mixing the chlorine with the pulp, reacting the chlorine with the pulp not over five minutes, whereby the noncellulosic constituents are substantially completely chlorinated without appreciable efiect upon the cellulose, and thereupon introducing an alkali into the reaction mixture in amount sufficient to neutralize the mixture and combine with the unconsumed chlorine, thoroughly mixing the alkali with the pulp, reacting the resulting hypochlorite 10 with the pulp and washing the pulp.

JOHN D. RUE. 

